The present invention relates to a single-tube color television imaging apparatus having a field memory in which the beam deflection system's nonlinearity information is stored as a reference signal for generating primary color video signals.
In single-tube color television imaging apparatus, a color filter is affixed to the photoelectrical conversion target of the image pickup tube to generate a color-multiplexed video signal.
Various proposals have been made in the arrangement of color filter stripes and in the circuit configuration that enables detection of primary colors. One such color filter comprises a plurality of successively arranged, recurrent groups of filter stripes of different colors. When the target is illuminated with light of uniform intensity having one of the colors of the filter stripes, an electron beam scanning the target generates a constant-amplitude signal at a constant frequency equal to the spatial frequency of the recurrent groups of the filter stripes. This signal is equivalent to a "carrier" as used in conventional modulation systems. When the target is illuminated with an actual color image, the "carrier" is modulated in amplitude in accordance with the intensity of picture elements and in phase in accordance with the arrangement of the individual stripes of the color filter. The resultant signal contains DC and luminance components, a fundamental frequency component having the same frequency as the carrier and a second harmonic component.
In copending U.S. patent application No. 582,226, the fundamental frequency component is detected from the color-multiplexed video signal by means of a pair of synchronous detectors using a pair of reference phase signals. To compensate for nonlinearity inherent in the beam deflection system, the reference phase signals are derived from a field memory in which the "carrier" of a field interval is stored. The outputs of the synchronous detectors are applied to a color matrix where they are combined with the DC component of the color-multiplexed signal to derive signals of the three primary colors.
It is known that individual color components can be detected with a high signal-to-noise ratio if the inputs to a color matrix include as many frequency components of the color-multiplexed signal as possible. However, due to undesirable factors including the finite size of beam spot, a tendency of the "carrier" modulation index to decrease at high frequencies, and a distribution of various components of noise generated by the image pickup tube, the harmonic components of the color-multiplexed signal are more contaminated with noise than the fundamental component is. For this reason, a further improvement in signal-to-noise ratio has been desired. One approach to this problem is to employ a color filter of the type having recurrent groups of magenta, cyan and yellow filter stripes. However, a satisfactory level of luminance signal cannot be obtained.